The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as of the priority date of the application.
Pressure swing adsorption (PSA) is an established technology employed to separate gas mixtures by exploiting differences in the adsorption capacities of the constituent gases, or, in a few industrial examples, by exploiting differences in sorption rates to achieve a kinetic selectivity. A continuous adsorption process can be realised with multiple beds operating in cycles of high-pressure adsorption steps and low-pressure regeneration or desorption steps together with intermediate bed pressurisation steps. Such a stripping-type PSA process can be designed by selecting an appropriate adsorbent, operating pressures, flow regime and step durations to remove methane from nitrogen or air to a very low concentration (e.g. <100 ppmv). However, one limitation in the application of PSA to gas purification is that few PSA designs can produce both a high-purity light product (raffinate) from the adsorption steps and an enriched secondary product (extract) during the regeneration steps.
The dual reflux PSA (DR-PSA) processes (also known as duplex PSA) combines the stripping and enriching PSA cycles into a single system with at least two beds where the feed gas is fed to an intermediate position along the adsorbent bed and some of each product stream is refluxed into the respective ends of the two adsorption columns. Dual reflux PSA processes have features similar to a continuous distillation process; for example when both gas species are competing for adsorption sites the phase equilibria (vapor-solid) is akin to vapor-liquid equilibria, and the use of light and heavy refluxes through the adsorbent beds is similar to the use of vapor and liquid refluxes in a distillation process.